The invention relates to electro-welding, or electro-fusing, elements made of plastic material, especially thermoplastics, for welding those elements together.
Apparatuses for electro-welding such elements are already known.
Typically, such apparatuses comprise:
a power source for generating an electrical signal, and for having an AC current in circulation,
an electrical converter AC/DC,
heating means, electrically connected to the converter for heating the weldable elements, at least locally, up to a temperature which is sufficient for fusing those elements and welding them, together.
Such an apparatus is notably disclosed in FR-A-2 572 326 or U.S. Pat. No. 5,138,136.
FR-A-2 572 326 relates to the automatic welding of plastic components including an integrated winding (coil). In order to carry out an automatic welding operation taking into account the diversity of the components used, an identification card, including the parameters useful for the production of the weld is associated with the components to be welded and means for reading the said card and for initiating the specified welding program, whilst taking into account the temperature of the components measured by a thermal probe, are provided on the welding machine. The invention is particularly applicable to the production of gas distribution networks by means of plastic pipes.
A system for making electrofusion joints between pipework components of thermoplastics materials containing resistance windings as inserts incorporates a device programmed to select the heating cycle appropriate to any particular combination of components, and compensate as necessary for variations in their initial temperature, in controlling the power supply and consolidation pressure cycles for effecting the joint. Each component is provided with a label or tag giving all necessary details about the component in a form suitable for automatic transcription into the programmed device, e.g. by optical or magnetic scanning.
However, the welding apparatuses of the prior art are not designed for being efficiently adapted to outdoor working conditions, especially in relation to the temperature.
Another problem is to be solved for adapting the welding apparatuses to the various elements to be welded, in relation to the compatibility between the apparatuses provided with one electrical resistance (such as an electrical resistance embedded within an electro-weldable connector made of plastic materials) and those apparatuses which are provided with heating means adapted for being heated by induction.
A further problem which is to be solved by the present invention relates to designing the welding apparatus so that it can operate as well as an  less than  less than inverter greater than  greater than  (supplied by a battery for having a direct, or unidirectional, DC current in circulation), or as an induction welding apparatus.
For improving the operating conditions of the existing apparatus and for solving at least a portion of the above-mentioned problems, an important feature of the invention consists in interposing electrical regulating means between the electrical converter and the heating means, those electrical regulating means being subjected to the commutating (or the switch-over) rhythm of switching means (also called  less than  less than switch greater than  greater than ) for generating a commutated electrical signal having a cyclic ratio (xcex1) which is variable, with
xcex1=URMS2/UDC2
and
xcex1=t2/(t1+t2),
UDC: input electrical voltage of the regulating means (voltage just upstream the regulation),
URMS: variable output electrical voltage of the regulating means corresponding to the electrical current supplied to the heating means,
t1: time interval for each non-passing phase of the switch,
t2: time interval for each passing phase of the switch.
For enlarging the use of the welding apparatus of the invention and avoiding difficulties in electrically supplying said apparatus with electrical energy, another feature of the invention relates to interposing an electrical accumulator between the DC output of the converter and the regulating means.
Further, for improving the efficiency and reliability of the electro-welding (temperature, swiftness . . . ) during the fusing phase, another feature of the invention recommends that the variable output voltage of the regulating means is a stepped (or crenelled) voltage which is periodically null, during each non-passing phase (t1) of the switch.
Furthermore, especially for allowing the welding apparatus of the invention to operate for heating by induction the elements to be welded, another feature of the invention recommends that the commutating rhythm (xcex4 period) of the switching means is variable, so that the electrical signal frequency delivered to the heating means is also variable, together with the heating frequency.